Dielectric of condensers and the insulation of cables and other conductors



Feb. 19, 1929. 1,702,993

S. G. BROWN DIELECTRIC OF CONPENSERS AND THE INSULATION OF CABLES ANDOTHER CONDUCTORS Filed Jan. 27, 1928 ofgwd Patented Feb. 19, 1929.

UNITED STATES PATENT OFFICE.

SIDNEY GEORGE BROWN, OF NORTH ACTON, ENGLAND, ASSIGNOR 0F ONE-HALF TOTHE TELEGRAPH CONDENSER COMPANY LIMITED, OF NORTH ACTON, ENGLAND, A

BRITISH COMPANY.

mnnnc'rnre or connnnsnns AND THE INSULATION or CABLES AND ornnneonnnc'rons.

Application filed January 27, 1928, Serial No. 249,916, and in GreatBritain January 19, 1927.

This invention relates to dielectric of c011- densers and the insulationof cables and other conductors carrying high tension current orsubjected to large differences of potential. The chief object is todecrease power losses and to prevent the eventual breakdown of thedielectric by long continued action of high potential differencesbetween the two sides of the insulating layer. It has been found thatafter use for a substantial period the insulating layer, commonlyobtained by oil saturated material, such as paper or the like,deteriorates and is liable to break down completely. I have found thatsuch deterioration is caused by the action of a voltage exceeding apredetermined limit and below such voltage the insulation maintains itsefficiency, the actionbeing apparently due to an ionizing effect of thevoltage upon the material of the insulator associated with a smallamount of air trapped in the material, but the method adopted accordingto this invention is independent of any explanation of the action whichis found to take place. The method employed isso to divide thedielectric into two or more layers (according to the voltage to which itis to be subjected) by an isolated conducting layer or layers that thevoltage across any one section of dielectric is kept below the level atwhich thedestructive ionizing action on the insulation is found to beoperative. With a dielectric such as is used in ordinary condensersemploying oil or wax saturated paper the ionizing potential has beenfound to be in the neighbourhood of 250 to 800 volts, a potentialdifferences substantially above the level causing more or less rapiddeterioration of the insulating properties, with danger of a finalcomplete breakdown. With such dielectric for the condenser or hightension cable the insulation should therefore be divided up by theinterposed conducting layers so that from one layer to the next thepotential should not to 300 volts.

There have been various proposals to provide the dielectrics ofcondensers for high potential use with interleaving of conductingsheets, but such interleaving has not been such as is directed to thelimitation of the voltage across any dielectric division to a levelbelow ionization and the present invention is based upon the discoverythat there is exceed, say, 250

a definite limit below which the voltage must be kept, in order that thefinal breakdown of the ielectric in high voltage condensers or cablesshall not take place owing to continuous ionizing action. In previousproposals interleaving of the dielectric has been suggested for thepurposes of more uniformly loading the dielectric and to avoid atendency towards brush or glow discharge. Such discharge occurs only atvoltages far above that contemplated in the present invention, in whichthe fall of potential across a single dielectric section is below 300volts.

The condenser according to this invention may be of plate form orrolled, in the latter case one ormore isolated conducting strips beingrolled up with the insulating and the ordinary or active conductingstrips constitnting the condenser. The isolated conductor may be made bycoating a strip or sheet of paper or other insulating material withfinely divided carbon, preferably in the form of vegetable black. Carbonin this form, owing to the fineness of its particles, forms an extremely thin continuously conducting film. One or both sides of one ormore of the usual dielectric sheets or strips of a rolled or othercondenser, or layers of insulation of a cable or other conductor, can becoated in this manner. The isolated conducting strips or layers mayextendbeyond the ordinary conductors to give greater protection to thelatter.

In order that the said invention may be clearly undersood and readilycarried into effect the same will now be more fully de-.

scribed with reference drawings in which Figures 1, 2 and 3 illustratein a diagrammatic form three types of condenser embodying thisinvention; and

igures 4 and 5 are cross sections of two forms of cable illustrating theuse of. the invention in connection with the insulation of high voltagepower transmission cables.

Referring to Figure 1, which illustrates a flat or platecondenser, A, Aare the active condenser plates connected by bridges a. B, B are thedielectric plates and C, C are the isolated conductors, in the form ofthin plates or flat sheets with conducting surfaces.

Figure 2 illustrates a rolled type of condenser with the interleavedstrips shown parts 1y unrolled to facilitate illustration. A, A

to the accompanying are the usual or active conducting strips which maybe of metal foil, separated by the insulating strips B of paper. C, arethe isolated conducting strips rolled up with the other components ofthe condenser.

In Figure 3 the rolled type of condenser is also illustrated, theisolated conducting strip C being shown as of greater width than theusual active strip conductor A The- B with integleavcd isolated windingsC in such number as to bring the voltage across any single layer of thepaper or other insulation B below the ionizing potential.

In Figure '5 a three phase cable is illustrated, with three conductors Din a single sheath E and each surrounded by interleaved windings B Cbetween the three covered conductors being insulation B of any suit abletype, filling up the space within the sheath.

The isolated conducting layer B, B or B need not in most cases be one ofhigh conductivity so long as it is suflicient to maintain the potentialdifference across the dielectric layer below the danger limit. Theconducting layer, may for example, be in the form of thin metal foil orfine wire, or may even (2011- sist of a surface layer of graphite oramorphous carbon, preferably in the form of vegetable black, on one orboth sides of the paper sheet or sheets formingthe basis of theinsulator 0r dielectric, if the paper 'niethoglof formation is employed.layer is independent of any outside connec tion and its potential woulddepend entirely upon the potential difference between the plates of thecondenser or the two conductors of the cable (core and sheath) 'atopposite sides of the dielectric. It has been found that the mereincrease in thickness of the insulator or dielectric does not greatlyincrease the potential difference which can permanently be'employed, ahigh tension across any practicable thickness of dielectric finallycausing breakdown by the ionizing action. The only method therefore toprevent such action taking place is to maintain the difference ofpotential across any layer of dielectric below the ionizing potentialand this can be effected by a conducting layer which separates thedielectric although carrying no substantial current.

\Vhat I claim and desire to secure by Let-.

ters Patent of the United States is i- 1. An insulated conductor havinglayers of paper and a film of finely divided carbon interposed betweenthe layers.

2. An insulated conductor having layers of paper and a film of vegetableblack interposed between the layers.

SIDNEY GEORGE BROWN.

The conducting

